A major problem in the manufacture of semiconductor materials by molecular beam epitaxy (MBE) is the uncertainty in the exact composition and fluxes of constituents used to grow a successful crystal. Beam fluxes can be controlled by opening and closing mechanical shutters and by adjusting the temperatures of oven sources or by controlling the flow rate of metal organic vapor sources. However, direct monitoring of the gaseous species that actually impinge on the substrate, including impurities from cross-contamination between oven sources, is not readily possible. The ability to monitor, nonintrusively and directly in real time, the incoming and scattered chemical fluxes within a few millimeters of the substrate would result in greater control of the growth process, as well as increase reproducibility of material quality.
Methods currently available for monitoring MBE growth provide limited flux information. For example, quartz crystal microbalances and nude ionization gauges provide quantitative flux values but give no species specific information. An electron impact emission source provides quantitative, species specific measurements, but it is not easily applied to simultaneous monitoring of multiple species and does not measure the actual fluxes impinging on the substrate. Hollow cathode discharge lamps can be used to perform either emission or absorption spectroscopy on the chemical beams, but this technique is also hindered by difficulties when it is required to monitor multiple species simultaneously. Reflection mass spectroscopy (Tsao et al.(1989) Appl. Phys. Lett. 55:777) can also be used to monitor fluxes, but it has the drawback of not being able to monitor incoming fluxes to the substrate. The resultant measurements are also hindered by dissociation of detected species in the normal electron impact mass spectrometer ionizer, which complicates the interpretation of the signals.
The actual growth of the material can be monitored directly by such techniques as reflection high energy electron diffraction (RHEED). RHEED monitors the progress of the growth but does not give information regarding the density and composition of the impinging gaseous beams. A more ideal situation would be to have complementary, noninvasive methods to monitor multiple species quantitatively and in real time from the impinging chemical beams while simultaneously monitoring the growth on the substrate with techniques such as RHEED and ellipsometry. Tsao, supra, uses RHEED analysis simultaneously with reflection mass spectrometry.
Another category of relevant related art consists of studies utilizing photon ionization in conjunction with time-of-flight mass spectroscopy (TOF-MS). Single photon ionization (SPI) by 118 nm light, generated by frequency tripling the 355 nm frequency tripled output of a Nd:YAG laser, has been used to study organic molecules such as simple alkanes (Steenvoorden et al. (1991) Int. J. Mass Spectrom. Ion Processes 107:475), tripeptides (Becker et al. (1990) Int. J. Mass Spectrom. Ion Processes 95:R1), and various organic polymers (Pallix et al (1989) Anal. Chem. 61:805). Studies by Brum et al. (1990) Appl. Phys. Lett. 56:695 and Alford et al. (1991) J. Chem. Phys. 94:2618 used 193 nm light from an ArF excimer laser to detect trimethylaluminum and silicon clusters, respectively.
A patent by Becker et al. (1988) U.S. Pat. No. 4,733,073 describes a technique for analyzing solid samples using a desorption probe beam, nonresonant photoionization of desorbed material, and TOF-MS. In Becker's apparatus, the mass spectrometer inlet is positioned opposite the sample surface and ions are extracted perpendicular to the sample surface.
Chien and Sogard (1990) J. Vac. Sci. Technol. A 8:1597 describe a method using multiphoton ionization (MPI) and TOF-MS to measure beam flux in a MBE system. In their apparatus the direction of the molecular beam is perpendicular to both the laser beam and the axis of the mass spectrometer. They demonstrate monitoring of molecular beam fluxes but do not insert a sample substrate in their apparatus.